1. Field of the Invention
The present invention concerns a printing unit and a gasket wherein the printing unit has a doctor blade chamber and a rotatable roller, the roller extending partially into the doctor blade chamber for receiving ink from the doctor blade chamber during operation. For this is provided a gasket for sealing between the doctor blade chamber and the roller, where the gasket at one side has a sealing surface for sealing abutment against a bottom of the doctor blade chamber, and where the gasket a side opposite the sealing surface has an elastomeric edge for sealing abutment against the roller.
2. Description of Related Art
A rotary printing unit for colour printing on printed matter typically comprises a semi-open doctor blade chamber with ink which is transferred to a screen roller in that the screen roller while rotating runs with part of its surface submerged in the ink bath in the doctor blade chamber. In order to seal the semi-open chamber towards the ink roller, the doctor blade chamber is connected with a doctor blade bearing on the roller. The ends of the doctor blade chamber are sealed with rubber gaskets which are embedded in the doctor blade chamber with their bottoms and which have a curving top side in contact with the roller. Such rubber gaskets can also be used for dividing the doctor blade chamber into several sections with ink baths.
In European Patent Application EP 401 250, which corresponds to International Patent Application Publication WO 89/07047 A1, is disclosed a doctor blade device which is represented in FIG. 1. The device includes a chamber bar with a U-shaped doctor blade chamber 3 with bottom 3A and sides 3B and 3C which during operation contain ink for a printing unit with a screen roller (not shown) which is in contact with the ink in the chamber 3. Two doctor blades 1, 2 are clamped to the chamber 3 by rails 5, 6, having the function of sealing against the screen roller, its surface being in contact with the ink in the chamber 3. Ink can be conducted into the chamber 3 via channels 8. Two channels 8 are shown, one for each part chamber in the chamber 3, wherein the part chambers are provided by delimitation by means of a packing 4 inside the chamber and a packing 4 at the end of the chamber. The packing 4 has a concave shape 4A for bearing against the screen roller.
A packing of this type is reproduced in FIG. 2 which is a copy of the U.S. Design Pat. D 488,503. This packing has a central section 10 with a sharp rail 7 of a hard material for bearing against the screen roller for enhanced sealing. Packings or gaskets with rails for bearing against the screen roller are also described in International Patent Application Publication WO 2009/024151.
Various embodiments of supports for doctor blades have been attempted as well, for example straight edges 9 as shown in FIG. 2, or combinations of a straight edge 9 and a curving edge 9′ at each their end of the central section 10 as shown on FIGS. 3a and 3b. The second lip 9′ curves in against the central section 10, where at the transition point 15 it has tangential direction relative to the screen roller in order to support tangential contact of the sealing doctor blade on the rotary roller. The curvature of the lip 9′ is increased from the outer edge 11 where the sealing doctor blade leaves the holder, in direction against the central section 10. With reference to the auxiliary line 12, it appears that the curvature actually changes such that the curvature is lesser in the second half 14 of the lip 9′ than in the first half 13 of the lip 9′. A further improvement of the course of the doctor blade is described in International Patent Application Publication WO 2010/060433 where the sealing doctor blade is elastically deformed into an arch with a curvature which is greater at the holder than at the contact area on the rotary roller.
In general, long life of the sealing and the tightness between the doctor blade chamber and the roller are important aspects and subject to continuous improvement; and as it appears from the above description of prior art there are various initiatives with regard to the course of the doctor blade as well as to the design of the gaskets. For example, it is desirable with improved durability of the gaskets. Even though the above described hard rails in the gaskets/packing are an improvement compared with the prior art, these gaskets have the drawback that the colour pigments are wearing considerably on the hard rails. Making these rails harder may reduce wear but implies a risk of more rapid wear on the screen roller at the line of contact with the rail.